This invention relates generally to television receiver color tuning systems and in particular to a television receiver color tuning system.
There are many known varieties of automatic color tuning systems for television receivers. A conventional color tuning system includes a color oscillator that operates at a nominal frequency of 3.58 MHz, which is controlled or locked up with the color burst signal transmitted with the received color television signal. Generally a form of phase locked loop is used for this purpose with the loop gain being selected based upon a compromise between the divergent requirements of pull-in frequency range and speed of response. In automatic color tuning systems, the color oscillator is caused to sweep, that is, to change its frequency within a limited frequency range, for locking up the color burst.
Prior art automatic color tuning circuits that continuously sweep the oscillator frequency when color is undetected, produce interference patterns on the cathode ray tube. The interference pattern has a general "herringbone" configuration and is quite disconcerting to a viewer. With the circuit of the invention, sweeping of the color oscillator is minimized and therefore such distortion is minimized. Further, the color oscillator of the inventive circuit is always tuned closely to the color burst, when the color burst is present, and therefore requires a minimum offset voltage. This, as is wellknown in the art, minimizes phase errors and broadens the "pleasing flesh tone" tuning range of the receiver.